MC9S12 Hardware initialization

This MCU has been used in our product over 5 years, however, a trouble was found months ago.

This product serves as a communication board, in this board the MCU has been designed to be reset with a hardware watchdog Supervisory IC(MAX705ESA). The COP function of MCU is not active.Once the assigned WDI(Watch Dog Input) pin on MCU was set to output, the Supervisory IC will work. WDI will periodically(we use 500ms) export a 5V signal to inform Supervisory IC that MCU is all right. If the MCU pin WDI was set to output, and it does not export 5V signal periodically, Supervisory IC will generate a low-level voltage to reset the MCU.

A problem was found when we using CAN measurement function, which means there were larger CAN messages interaction on CAN bus(we use CAN0 and CAN4)，the MCU stopped and the watchdog didn't reset the MCU successfully. After power-on again, the fault disappears.

More than one case was revealed and we have monitored the watchdog IC, the WDI pin was not set to output. I guess the MCU has ran to initial state and stopped. One possibility was that MCU ran to exception that MCU pins become high-impedance state, or the MCU has been reset and it has not started successfully.

I want to know what can lead to this situation that MCU pins become unusual just like this case.